One of the ongoing problems in the theatre is how to secure props and scenery elements on the stage quietly and quickly and to then be able to move them also quietly and quickly.
One method for doing this with large scenery elements and props is to move them with an automatic winch. This is very expensive. It is also impractical when more than one production might be performed on the same stage during the same season.
Smaller prop units can be mounted on casters, but these are harder to stop effectively in a fixed position and orientation. Wedges and cane bolts may be used but they are ineffective and awkward. Step on caster brakes are also ineffective. Pianos slip and sofas slide.
A caster used for moving scenery elements is a “swivel caster”. A swivel caster comprises two side by side wheels. The side by side wheels reduce “swing wiggle” when the direction of movement of the prop is changed. Swing wiggle refers to the unintended movement of the prop due to the changing orientation of the caster wheels when the direction of movement is changed. This is particularly pronounced when a prop is first moved in one direction and then moved in the opposite direction. Not only does the prop wiggle, but the force required to reverse the motion of the prop can be very high. Swing wiggle must be minimized for prop set up and striking and for props that are moved during a performance, such as props that are danced with.
Another caster used for scenery elements is the “triple swivel caster”. A prior art triple swivel caster 600 is illustrated in FIG. 6. This caster is sold by Centerline Studios of Cornwall N.Y. The caster is more fully described on web page www.fasor.com/centerline/tri_swiv.html (last viewed Jul. 31, 2002). The triple swivel caster comprises three swivel casters 601, 602, 603, mounted on bottom plate 604. The bottom plate is swivel mounted on top plate 605. The top plate would be attached to the prop or scenery elements that are to be moved. Triple swivel casters are used for heavy props, such as pianos, sofas and the like. Triple swivel casters have very little “swing wiggle”.
Portable locking mechanisms are known for casters. U.S. Pat. No. 4,998,320, “Apparatus for Movably Supporting Beds and the Like” by Hans-Willi Lange, describes a portable pneumatic air braking system for single casters mounted on a hospital bed frame. The rolling and swivel motion of the casters are normally prevented due to a spring loaded brake applied thereto. When it is desired to move and rotate the hospital bed, compressed air is pumped into the brake assembly thus forcing the brakes to disengage from both the wheels and the wheel frames. Air is pumped into the brake assembly with a hand pump. This system would not be suitable for a theatre prop due to the large and visible nature of the hand pump and the swing wiggle due to the single wheel casters.